The present invention relates to a method and apparatus for inspecting the surface of an article. The present invention has particular applicability for in-process inspection of semiconductor wafers during manufacture of high density semiconductor devices with submicron design features, and for inspection of reticles used for semiconductor device manufacture.
Current demands for high density and performance associated with ultra large scale integration require submicron features, increased transistor and circuit speeds and improved reliability. Such demands require formation of device features with high precision and uniformity, which in turn necessitates careful process monitoring, including frequent and detailed inspections of the devices while they are still in the form of semiconductor wafers.
Conventional in-process monitoring techniques employ an xe2x80x9cinspection and reviewxe2x80x9d procedure wherein the surface of the wafer is initially scanned by a high-speed, relatively low-resolution inspection tool; for example, such a tool can include a laser and an opto-electric converter such as a CCD (charge-coupled device). Statistical methods are then employed to produce a defect map showing suspected locations on the wafer having a high probability of a defect. Typically, after a redetection procedure is carried out, using the defect map, to positively determine the presence of defects, a more detailed review procedure is carried out on the individual defect sites, such as with a scanning electron microscope (SEM) to produce a relatively high-resolution image. The defect image is then analyzed to determine the nature of the defect (e.g., a defective pattern, a particle or a scratch).
Current laser inspection techniques typically scan the wafer under inspection with laser light, and detect scattering or diffraction of the incident light by structures and defects on the wafer surface. In other words, information produced by the scattering or diffraction of light at the incident wavelength, known as xe2x80x9clinear optical phenomenaxe2x80x9d, are used to determine whether or not defects exist on the wafer surface. Linear optical phenomena are affected largely by the geometry and refractive indices of the materials on the wafer, and therefore typically yield information regarding the size and shape of surface features. This information from a site under inspection is compared with linear optical phenomena observed at a nominally identical site on the wafer or a reference site, and if the two sites do not exhibit the same optical phenomena, it is determined that a defect may exist at the inspected site.
Conventional laser inspection techniques have several shortcomings. They do not gather information directly relating to the material composition of surface features or defects that would indicate the presence of an unwanted foreign material. Additionally, they typically do not reliably detect small defects that scatter a limited amount of light, since the light scattered by a small defect can be lost in the light scattered by features, such as patterns, on the wafer surface proximal to the defect. Thus, a small defect may not be detected at all, depending on its location on the wafer surface. The limited accuracy and scope of conventional laser inspection lowers manufacturing yield and increases production costs.
Further information is obtainable from a laser inspection of wafer surface, especially information relating to chemical composition and interfaces between materials, by observing nonlinear optical effects, broadly defined as those in which the radiation emanating from the illuminated region of the wafer contains wavelengths other than that of the incident radiation. Potentially useful nonlinear optical phenomena include photoluminescence (also known as xe2x80x9cfluorescencexe2x80x9d), Raman scattering and second harmonic generation. However, although fluorescence microscopes and scanning Raman microscopes are commercially available; e.g., for studying biological samples and contaminants at the defect review stage, such systems are not suitable for high-throughput semiconductor wafer inspection. They are not designed to scan an entire wafer, but rather to image isolated small regions, and their scan speed is incompatible with the throughput required of an automated wafer inspection system. Moreover, they do not incorporate the requisite wafer handling systems, defect detection electronics and algorithms.
There exists a need for a methodology and apparatus for in-process inspection of semiconductor wafers that provides information relating to the material composition and interfaces between materials of surface features and defects. This need is becoming more critical as the density of surface features, die sizes, and number of layers in semiconductor devices increase, requiring the number of defects to be drastically reduced to attain an acceptable manufacturing yield.
An advantage of the present invention is a method and apparatus for optical inspection of semiconductor wafers that obtains information relating to the chemical composition and interfaces between materials simultaneously with obtaining topographical and feature size information, thereby enabling fast, reliable and comprehensive defect detection.
Additional advantages and other features of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The advantages of the invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by an apparatus for inspecting a surface of an article, the apparatus comprising a light source for irradiating a portion of the surface of the article with a light beam at an incident wavelength; a first detector for receiving light at the incident wavelength from the portion of the surface and generating a first signal; a second detector for receiving light at a wavelength different from the incident wavelength from the portion of the surface and generating a second signal; and a processor configured for determining, based on the first and second signals, whether a defect exists on the portion of the surface.
Another aspect of the present invention is a method for inspecting a surface of an article, the method comprising irradiating a portion of the surface of the article with a light beam at an incident wavelength; receiving light at the incident wavelength from the portion of the surface at a first detector to generate a first signal; receiving light at a wavelength different from the incident wavelength from the portion of the surface at a second detector to generate a second signal; and determining whether a defect exists on the portion of the surface based on the first and second signals.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.